Standards for Superconductor Characterization

Project Goals

Lowering a test fixture into a critical-current measurement cryostat. The cloud at the top of the cryostat results from the boiling cryogen.

This project develops standard measurement techniques for critical current and provides quality assurance and reference data for commercial high-temperature and low-temperature superconductors. Applications supported include magnetic-resonance imaging, research magnets, fault-current limiters, magnetic energy storage, magnets for fusion confinement, motors, generators, transformers, transmission lines, magnets for crystal growth, and superconducting bearings. Project members assist in the creation and management of international standards for superconductor characterization covering all commercial applications, including electronics. The project is currently focusing on critical-current measurements of marginally stable superconductors, on temperature-variable critical-current measurements, and on measuring the irreversible effects of changes in magnetic field and temperature on critical current.

Customer Needs

We serve the U.S. superconductor industry, which consists of many small companies with limited resources for committing to the development of new metrology and standards. We participate in projects sponsored by other government agencies that involve U.S. industry, universities, and national laboratories.

The potential impact of superconductivity on power systems makes this technology very important. We focus on:
(1) developing new metrology needed for evolving, large-scale superconductors.
(2) participating in interlaboratory comparisons needed to verify techniques and systems used by U.S. industry.
(3) developing international standards for superconductivity needed for fair and open competition and improved communication.

Technical Strategy

This project's primary activities are critical-current measurement metrology, interlaboratory comparisons, and development of international standards. One of the most important performance parameters for large-scale superconductor applications is the critical current. Critical current is difficult to measure correctly and accurately; thus, these measurements are often subject to scrutiny and debate.

With each significant advance in superconductor technology, new procedures, interlaboratory comparisons, and standards are needed. International standards for superconductivity are created through the International Electrotechnical Commission (IEC), Technical Committee 90 (TC 90).

The next generation of Nb3Sn and Nb3Al wires is pushing towards higher current density, less stabilizer, larger wire diameter, and higher magnetic fields. The resulting higher current required for critical-current measurements turns many minor problems into significant engineering challenges. For example, specimen heating, from many sources, during the measurement can cause a wire to appear to be thermally unstable.

Milestones

During 2001-2003, continue to manage and help create international standards for superconductivity.

In 2001, study the effect of extrinsic parameters on overall stability of Nb3Sn wires during routine testing to 1000 amperes in a magnetic field of 12 teslas.

By 2002, determine a procedure for routine critical-current measurements up to 1500 amperes at fields of 12 teslas.

In 2001, complete a detailed report on variable-temperature critical-current measurements of high-temperature superconductors.

Accomplishments

Critical-Current Tutorial

We gave an invited tutorial during the 2000 Applied Superconductivity Conference on critical-current measurements for power applications. The procedure consists of measuring the voltage-current characteristic and applying a criterion to determine the critical current. Typical criteria are electric-field strength of 10 microvolts per meter and resistivity of 10-14 ohm meters.

Performance of Wire Verified

A national laboratory involved in the U.S. effort for the international Large Hadron Collider program asked us to conduct critical-current verification on several Cu/Nb-Ti wires. The critical-current specification was 1925 amperes at 5 teslas. The current density in the Nb-Ti is over 3000 amperes per square centimeter. These wires were designed with only the minimum amount of Cu stabilizer, which made them difficult to measure. The application will use a cable constructed of these wires and additional high-purity aluminum stabilizer will be added to the cable to make the application more stable. We tested 44 specimens from about 528 km of wire so far in the program. The distribution of critical currents is shown in the plot below. Most of the specimens tested above the specification. The lowest value on one specimen was found to be caused by an end effect. Our resistivity measurement at room temperature indicated that the copper fraction of this specimen was higher than in all the other specimens, which is consistent with a lower critical current. With this observation, the manufacturer was able to crop one end of the wire. A new specimen was tested and found to be above the critical-current specification, which saved about 11 kilometers of wire from rejection. This is one example of how we help solve some of the measurement problems facing the U.S. superconductor wire industry. Our experience with such measurements will help in making future standards more practical.

Variable-Temperature Measurements of Critical Current

We recently developed and tested a new variable-temperature critical-current measurement system. We used this system to measure commercially produced Ag-sheathed Bi2Sr2Ca1Cu2Ox and Bi2Sr2Ca2Cu3Ox multifilamentary tapes to currents over 250 amperes. The critical currents of these high-temperature superconductor tape samples depend on the angle of the applied magnetic field relative to the c-axis of the material.

Sixth IEC Meeting on Superconductivity

We organized, hosted, and participated in the sixth meeting of IEC Technical Committee 90 (TC 90) on superconductivity in Boulder, Colorado. The meeting allowed experts from around the world to come together and work to advance the draft international standards in six Working Groups. In addition, results from pre-standards research and interlaboratory comparisons were presented on four new working drafts and on six future drafts. To date, two IEC measurement standards have been published and a third standard on superconductivity vocabulary is being published. As a result of this meeting, three more draft standards are now ready for the final voting stage. Information about the IEC TC 90 activities can be found here (enter "90" for the committee number) or direct to IEC/TC 90.

Resistivity of High-Purity Cu

We measured residual resistivity ratio (RRR) measurements of four high-purity Cu samples (for a total of 35 for this program) from an aerospace company. The RRR measurements are used to estimate the low temperature thermal conductivity of thermal straps to be used in several instruments for the Space Infrared Telescope Facility.

Critical Current of Nb-Ti

We measured critical currents of three Nb-Ti conductors from a U.S. wire manufacturer. The currents required were beyond the capabilities of their measurement facilities. The company has an occasional need for some measurements at high current (2000 amperes) or high field (12 teslas). Ours is one of the few labs in the country that can do these measurements.

Suppression of Flux Jumps

We demonstrated that flux jumps could be suppressed during the measurement of hysteresis loss by immersing marginally-stable Nb3Sn conductors in liquid He. The increased thermal conduction affords dynamic stability against flux jumps, which allows ac losses to be estimated from the area of the magnetization-versus-field loop.

Standards Committees

Loren Goodrich was the Chairman of IEC/TC 90, the U.S. Technical Advisor to TC 90, the Convener of Working Group 2 (WG2) in TC 90, the primary U.S. Expert to WG4, WG5, and WG6, and the secondary U.S. Expert to WG1, WG3, and WG7. Ted Stauffer was Administrator of the U.S. Technical Advisory Group to TC 90.